1. Field of the Invention
This invention relates in general to a method and apparatus for obtaining phase in-sensitive and/or polarization-insensitive optical heterodyne reception for a FSK-modulated transmission signal.
2. Description of the Prior Art
An optical heterodyne reception in which a supplied optical transmission signal is superimposed with an optical local oscillator signal and the output signal of the receiver is obtained from the superimposed optical signals by way of an intermediate frequency filter. The receiver sensitivity is generally dependent on the difference between the polarization conditions and the phases of the transmission signal and the local oscillator signal. When the polarization conditions of the transmission signal and the local oscillator signal do not coincide loss in sensitivity occurs during reception. When a low intermediate frequency which is one or more orders of magnitude lower relative to the data rate transmitted by the transmission signal, the optical signals superimposed on each other also periodically change between in phase and quadrature phase components.
Prior art known polarization diversity receivers provide that the transmission signal is divided into two polarization conditions which are orthogonal relative to each other and these are separately received and electrically added to obtain polarization-insensitive reception.
Phase diversity receivers wherein the co-phasal and quadrature components are separately received and electrically added can be employed for phase and sensitive reception.
These prior art receivers require a 90.degree. hybrid which is difficult to realize in optical systems and also required two or four optical heterodyne receivers.
Phase-insensitive and/or polarization-insensitive receivers could also be obtained with a phase-control and/or polarization-control. However, such controls are extremely complicated.